1. Field of the Invention
The present invention relates to an information reproducing apparatus for reproducing digital information recorded on a DVD-RAM (DVD random Access Memory) by a single spiral land/groove recording method.
2. Description of the Related Art
A DVD-RAM is a recording medium whose storage capacity is about four times as large as that of a CD (Compact Disk), and it is a recordable recording medium which enables a user to rewrite information thereon several times.
FIG. 1 shows the recording surface of the DVD-RAM 1. As shown in FIG. 1, a land track 1L and a groove track 1G are formed in a spiral form on the recording surface of the DVD-RAM 1. The land track 1L and the groove track 1G are alternately arranged in the radial direction of the DVD-RAM 1.
Digital information is recorded on such a recording surface according to a predetermined recording format based on a DVD standard. A xe2x80x9csingle spiral-land/groove (SS-L/G) recording methodxe2x80x9d is standardized as one recording format of the DVD-RAM in the DVD standard. This method is described in a paper: xe2x80x9cAccessing method for single spiral-land groove recordingxe2x80x9d by Nakane et al., published in Technical Report of IEICE (the Institute of Electronics, Information and Communication Engineers), MR95-88, PCM95-126 (February 1996).
The single spiral land/groove recording method is adopted as the recording format of the DVD-RAM 1 shown in FIG. 1. In the single spiral-land/groove recording method, the digital information is recorded on both of the land track 1L and groove track 1G.
The digital information to be recorded on the DVD-RAM 1 is divided into sectors. The amount of the divided digital information contained in each sector is predetermined. Each of the land track 1L and groove track 1G are also divided into sectors corresponding to the sectors of the digital information, as shown in FIG. 1.
Control information recording areas S0-S7 are formed at the respective boundaries between neighboring sectors, as shown in FIG. 1. Hereinafter, the control information recording area is referred to as a xe2x80x9cCIR areaxe2x80x9d. The CIR areas S0-S7 are located at equiangular intervals in the direction of the rotation of the DVD-RAM 1.
At the CIR area S0, the arrangement of the land track 1L and groove track 1G are switched over to each other in the radial direction of the DVD-RAM 1. Namely, at the CIR area S0, the land track 1L is switched over to the groove track 1G, and the groove track 1G is switched over to the land track 1L. In other words, the land track 1L and the groove track 1G are substantially connected with each other through the CIR area S0. This structure enables the digital information to be continuously recorded onto or reproduced from both of the land track 1L and groove track 1G. In addition, at other CIR areas S1-S7, the land track 1L and groove track 1G are not switched over.
Control information is pre-recorded in each of the CIR areas S0-S7. The control information includes address information to substantially identify positions on the recording surface of the DVD-RAM 1. This information is needed for a recording process or a reproduction process of the digital information. For example, the control information includes information representing a physical position or a sector number to identify the sector located immediately after or before the CIR area.
FIG. 2 is enlarged view of a part of the recording surface of the DVD-RAM 1, which includes the CIR areas S0 and S1, and which is indicated by a broken line DL1 in FIG. 1. As shown in FIG. 2, the CIR area S0 is divided into two equal areas in the circumferential direction D1 of the DVD-RAM 1, which is shown by an arrow in FIG. 2. Each of the two equal areas is further divided into pre-recorded parts 70 and blank parts 71 in the radial direction D2 of the DVD-RAM 1, which is shown by another arrow in FIG. 2. Each of the pre-recorded parts 70 and the blank parts 71 is equal to the groove track 1G or the land track 1L in width (length in the radial direction D2). The pre-recorded parts 70 and the blank parts 71 are alternately located in each of the two equal areas in the radius direction D2. Each pre-recorded part 70 on the right side of the CIR area S0 is located xc2xd width out of the location of the groove track 1G in the radial direction D2. In contrast, each pre-recorded part 70 on the left side of the CIR area S0 is located xc2xd width out of the location of the groove track 1G in the radial direction D3 (opposite to the radial direction D2).
Control information is pre-recorded in each pre-recorded part 70 as pits P. In contrast, there is no pit on each blank part 71, so that the surface of each blank part 71 is like a mirror. This means that no information is recorded on each blank part 71. Each blank part 71 is equal to the land track 1L in height. The structure of each of the other CIR areas S1-S7 is the same as that of the CIR area S0.
When the digital information and the control information are read out from the DVD-RAM 1, a light beam is emitted from an optical pickup to the recording surface of the DVD-RAM 1. At this time, a light spot LS is formed by the light beam. The light spot LS is moved on the land track 1L and the groove track 1G alternately by the revolution of the DVD-RAM 1 and the movement of the pickup in the radial direction of the DVD-RAM 1. For example, the light spot LS is first moved on the groove track 1G. After the light spot has passed the CIR area S0, the light spot LS is next moved on the land track 1L. After the light spot LS has passed the CIR area S0 again, the light spot LS is next moved on the groove track 1G. In such a manner, the light spot LS is alternately moved on the groove track 1G and the land track 1L.
As shown in FIG. 2, the light spot LS is passed on an imaginary track T1 in the CIR area S0, when the light spot LS is moved from the groove track 1G to the land track 1L through the CIR area S0. When the light spot LS is passed on the imaginary track T1, the light spot LS is first passed on the pre-recorded part 70 located xc2xd width out of the location of the groove track 1G in the direction D3, and the light spot LS is next passed on the neighbor pre-recorded part 70 located xc2xd width out of the location of the groove track 1G in the opposite direction D2.
The light spot is passed on an imaginary track T2 in the CIR area S0, when the light spot LS is moved from the land track 1L to the groove track 1G through the CIR area S0. When the light spot LS is passed on the imaginary track T2, the light spot LS is first passed on the pre-recorded part 70 located xc2xd width out of the location of the land track 1L in the direction D2, and the light spot LS is next passed on the neighbor pre-recorded part 70 located xc2xd width out of the location of the land track 1L in the opposite direction D3.
The light spot is passed on an imaginary track T3 in the CIR area S1 (S2-S7), when the light spot LS is moved from a certain sector to the neighboring sector on the same groove track 1G. When the light spot LS is passed on the imaginary track T3, the light spot LS is first passed on the pre-recorded part 70 located xc2xd width out of the location of the groove track 1G in the direction D2, and the light spot LS is next passed on the neighbor pre-recorded part 70 located xc2xd width out of the location of the groove track 1G in the opposite direction D3.
The light spot is passed on an imaginary track T4 in the CIR area S1 (S2-S7), when the light spot LS is moved from a certain sector to the neighboring sector on the same land track 1L. When the light spot LS is passed on the imaginary track T4, the light spot LS is first passed on the pre-recorded part 70 located xc2xd width out of the location of the land track 1L in the direction D3, and the light spot LS is next passed on the neighbor pre-recorded part 70 located xc2xd width out of the location of the land track 1L in the opposite direction D2.
Now, it should be noted that the positional relationship between the two pre-recorded parts 70 adjacent to each other in the circumferential direction D1 is different between the imaginary tracks T1 and T3. On the basis of this difference, the change from the groove track 1G to the land track 1L can be detected. Similarly, the positional relationship between the two pre-recorded parts 70 is different between the imaginary tracks T2 and T4. On the basis of this difference, the change from the land track 1L to the groove track 1G can be detected.
FIG. 3 is a schematic view for showing the pickup 102 for reading the digital information and the control information from the recording surface of the DVD-RAM 1. The pickup 102 has an emitting device (not shown) for emitting the light beam to the recording surface of the DVD-RAM 1, and a detector (not shown) for receiving the light beam reflected by the recording surface. The detector has detecting surface 102A divided into two detection portions DP1 and DP2 by the tangential line of the groove track 1G (land track 1L). Namely, one detection portion DP1 is positioned above the outer circumferential side of the groove track 1G (land track 1L), and the other detection portion DP2 is positioned above the inner circumferential side of the groove track 1G (land track 1L).
The light beam reflected by the recording surface is received by the detection portions DP1 and DP2. The light beam received by the detection portion DP1 is converted to an electric signal. Also, the light beam received by the detection portion DP2 is converted to another electric signal. To obtain the digital information from the received light beam, a sum signal is generated by adding the two electric signals. To obtain the control information, a difference signal is generated by subtracting one electric signal from the other electric signal. The difference signal includes information indicating whether the location of the pre-recorded part 70 is off the location of the groove track 1G (land track 1L) in the radial direction D2 or D3. Therefore, the fact that the groove track 1G is switched over to the land track 1L or the fact that the land track 1L is switched over to the groove track 1G can be recognized on the basis of the difference signal.
FIG. 4 shows the sum signal Spp1 and the difference signal Spp2. In FIG. 4, the sum signal Spp1 has a relatively high frequency component. This component is a signal component necessary for reproduction, and contains the digital information. The amplitude of the sum signal Spp1 suddenly increases at time t1 and suddenly decreases at time t3, as a whole. Such sudden increase and decrease frequently occur whenever the light spot passes any of the CIR areas S0-S7. More specifically, the digital information is read out from the groove track 1G or the land track 1L during the time period t3-t4. During this time period, the middle level of the amplitude of the signal component of the sum signal Spp1 is L1. The control information is read out from any of the CIR areas S0-S7 during the time period t1-t3. During this time period, the middle level of the amplitude of the signal component of the sum signal Spp1 is L2. Thus, the middle level of the amplitude of the signal component of the sum signals Spp1 is varied depending on the reading position.
As for the difference signal Spp2, as shown in FIG. 4, the amplitude suddenly increases at time t1, and suddenly decreases at time t2, and the suddenly increases at time t3, as a whole. Such increases and decreases indicate whether the track on which the light spot is tracking is switched over from the groove track 1G to the land track 1L or from the land track 1L to the groove track 1G. Referring back to FIG. 2, when the light spot LS is passed on the imaginary track T2 in the CIR area S0, the light spot LS is passed firstly on the pre-recorded part 70 on the left side of the CIR area S0, and secondly on the pre-recorded part 70 on the right side of the CIR area S0. Therefore, as shown in FIG. 4, the amplitude of the difference signal Spp2 firstly increases and secondly decreases. On the other hand, when the light spot is passed on the imaginary track T1 in the CIR area S0, the amplitude of the difference signal Spp2 firstly decreases and secondly increases. By detecting such changes of the amplitude of the difference signal Spp2, the change from the groove track 1G to the land track 1L or the change from the land track 1L to the groove track 1G can be recognized. Furthermore, the difference signal Spp2 contains the relatively high frequency component during the time period t1-t3. This component is another signal component necessary for reproduction, and contains the control information. The middle level of the amplitude of the signal component of the difference signal Spp2 during the time period t1-t2 is L3. The middle level of the amplitude of the signal component of the difference signal Spp2 during the time period t2-t3 is L4.
In order to achieve reproduction of the digital information recorded on the DVD-RAM 1, it is needed to extract the digital information and the control information from the sum signal Spp1 and the difference signal Spp2, and to convert this information into binary data, respectively. However, the amplitude of each of the sum signal Spp1 and difference signal Spp2 is suddenly and frequently varied. Because of this, the middle level of the amplitude of the signal component including the digital information is different from that including the control information. In addition to this, the middle level of the amplitude of the signal component including the control information is suddenly varied at the time t2. These complications make it difficult to accurately extract the digital information and the control information and to accurately convert them into binary data. Therefore, there is a problem that it is difficult to enhance accuracy of the reproduction of the digital information from the DVD-RAM 1 using the single spiral land/groove recording method.
Meanwhile, there is another problem concerning to the extraction of the digital information from the sum signal Spp1. The sum signal Spp1 often contains noise components. The noise components are caused by disturbance, such as variations of reflectance of the DVD-RAM, variations of refraction factor of the DVD-RAM, an error of the servo mechanism to control a light spot position and the like. The frequencies of the noise components are relatively low, for example, less than about 100 kHz. On the other hand, the frequencies of the signal components including the digital information are within the range of about 100 Hz to 10 MHz. In the range of about 100 Hz to 100 kHz, both the noise components and the signal components are mixed. Therefore, the noise components cannot be sufficiently eliminated from the sum signal Spp1 by using an simple analog high pass filter, while maintaining the signal components in the sum signal Spp1. If the cut-off frequency of the analog high pass filter is set at about 100 kHz, the noise components can be sufficiently eliminated, but the signal components are partly lost. If the cut-off frequency of the analog high pass filter is set at about 100 Hz, the signal components can be maintained, but the noise components cannot be sufficiently eliminated.
The similar problem occurs concerning to the extraction of the control information from the difference signal Spp2.
Furthermore, in different aspect, it is not suitable to use an analog high pass filter for eliminating noise components from the sum signal Spp1. FIG. 5 shows wave forms each representing the signal component containing the digital information. In addition, the scale of the time base in FIG. 5 is different from that in FIG. 4. In FIG. 5, a wave form W1 represents an ideal wave form which corresponds to the pits formed on the DVD-RAM 1. A wave form W2 represents an actual wave form which corresponds to the pits and which is actually generated by the pickup. A wave form W3 represents an actual wave form obtained after the wave form W2 has been treated with the analog high pass filter. A wave form W4 represents an actual wave form obtained by converting the wave form W3 into binary pulse signal.
The pits formed on the DVD-RAM 1 have various lengths. The length of each pit depends on the digital information. Therefore, there is the case where several pits with long lengths are continuously formed on the track on the DVD-RAM 1, and next, several pits with short lengths are continuously formed on the track. In that case, in the wave form W1 shown in FIG. 5, several pulses with long widths P1 continuously appear during the time period T1, and next, several pulses with short widths Ps continuously appear during the time period T2. The broken line avg11 indicates the average of the amplitude of the wave form W1 during the time period T1. The broken line avg12 indicates the average of the amplitude of the wave form W1 during the time period T2. Now, it should be noted that the average of the amplitude of the wave form W1 during the time period T1 is higher than that during the time period T2.
As for the actual wave form W2 corresponding to the ideal wave form W1, the broken line avg21 indicates the average of its amplitude during the time period T1. The broken line avg22 indicates the average of its amplitude during the time period T2. Like the ideal wave form W1, the average of the amplitude of the wave form W2 during the time period T1 is higher than that during the time period T2. The dots on the wave form W2 indicate the zero cross points representing the intersection points of the sum signal Spp1 and the zero level in the amplitude.
As for the actual wave form W3 obtained after the wave form W2 is treated with the analog high pass filter, the broken line avg31 indicates the average of the amplitude during both of the time periods T1 and T2. The dots on the wave form W3 indicate the same zero cross points as those on the wave form W2.
As seen from FIG. 5, if the wave form W2 is treated with the analog high pass filter, the average of the amplitude of the wave form W3 becomes uniform, but the zero cross points are shifted downward (in the negative direction) during the time period T1. Further, during the time period T2, the zero cross points are shifted upward (in the positive direction).
If the wave form W3 is converted into the binary pulse signal on the basis of the average of its amplitude (avg31) in the following manner, the binary pulse signal having the wave form W4 is generated. Namely, it is determined whether or not the amplitude of the wave form W3 is higher than average (avg31); if so, then the level of the pulse signal is made high; if not so, then the level of the pulse signal is made low. As seen from FIG. 5, the pulse widths Plxe2x80x2 and Psxe2x80x2 in the wave form W4 becomes different from the pulse widths Pl and Ps in the original wave form W1. This means that the digital information is undesirably changed by the analog high pass filter. This causes accuracy of reproduction to worsen. A similar problem occurs in the difference signal Spp2.
Furthermore, there is further different problem. To increase a data transfer rate, a technique to change the reproduction speed of the digital information from the DVD-RAM is known. In this technique, the reproduction speed is changed by changing the rotational speed of the DVD-RAM. If the rotational speed of the DVD-RAM is changed, the sum signal and the difference signal are changed in frequency.
However, the cut-off frequency of the simple analog high pass filter is fixed. Therefore, noise components cannot be sufficiently eliminated from the sum signal or the difference signal in the case where the rotational speed of the DVD-RAM 1 is changed.
It is therefore an object of the present invention to provide an information reproducing apparatus wherein a detection signal having a plurality of different middle levels, which is to be used for reproduction, can be made into a detection signal having a single common middle level.
It is also an object of the present invention to provide an information reproducing apparatus which can eliminate noise components from a detection signal, while maintaining the component necessary for the reproduction contained in the detection signal.
It is further an object of the present invention to provide an information reproducing apparatus which can sufficiently eliminate noise components from a detection signal to be used for reproduction in the case where the reproduction speed is changed.
The above-mentioned objects can be achieved by an information reproducing apparatus in accordance with the present invention. The information reproducing apparatus is an apparatus for generating a detection signal corresponding to information recorded on a recording medium by detecting a light beam reflected by the recording medium, and for reproducing the information by decoding the detection signal. The detection signal includes at least a first component and a second component. The first component has a first middle level. The second component has a second middle level. The first middle level and the second middle level are different from each other.
The information reproducing apparatus includes: a light emitting device that emits the light beam to the recording medium; a detecting device that detects the light beam reflected by the recording medium, and that generates the detection signal including the first component and the second component; a sampling device that samples the first component and the second component included in the generated detection signal; an extraction device that extracts a first sampling value closest to the first middle level from the first component, and that extracts a second sampling value closest to the second middle level from the second component; a signal generation device that generates a first middle level signal by using the extracted first sampling value, and that generates a second middle level signal by using the extracted second sampling value; a compensation device that shifts the first component and the second component such that both the first middle level signal and the second middle sampling signal are matched to a predetermined level; and a decoding device that decodes the compensated first component and the compensated second component to reproduce the information.
In the information reproducing apparatus, the detection signal generated by the detecting device includes at least two component having different middle levels, namely, the first middle level and the second middle level. Such a detection signal is sampled by the sampling device. Then, the extraction device extracts a first sampling value closest to the middle level of the first component from the first component, and then, the signal generation device generates a first middle level signal by using the extracted first sampling value. The first middle level signal represents the actual middle level of the first component. If low frequency noise components are contained in the first component, the first middle level signal is undesirably varied from the normal middle level of the first component.
On the other hand, the extraction device also extracts a second sampling value closest to the middle level of the second component from the second component, and then, the signal generation device also generates a second middle level signal by using the extracted second sampling value. The second middle level signal represents the actual middle level of the second component. If low frequency noise components are contained in the second component, the second middle level signal is undesirably varied from the normal middle level of the second component.
The compensation device then shifts the first component and the second component such that both the first middle level signal and the second middle sampling signal are matched to a predetermined level. Therefore, the middle level of the first component becomes equal to the middle level of the second component, and as a result, the middle level of the whole detection signal is made uniform. Further, relatively low frequency noise components contained in the first component and/or the second component are sufficiently eliminated, while maintaining the first component and the second component.
This detection signal is decoded by the decoding device. Thus, the information recorded on the recording medium is accurately reproduced.
The extraction device may includes: a first value detection device that detects the first current sampling value and the first preceding sampling value, one of which is equal to or less than the first middle level, and the other of which is more than the first middle level, from the first component; and a first sampling value extraction device that extracts one closer to the first middle level from between the detected first current sampling value and the detected first preceding sampling value. Therefore, the first sampling value closest to the middle level of the first component can be extracted.
This extraction may further includes: a second value detection device that detects the second current sampling value and the second preceding sampling value, one of which is equal to or less than the second middle level, and the other of which is more than the second middle level, from the second component; and a second sampling value extraction device that extracts one closer to the second middle level from between the detected second current sampling value and the detected second preceding sampling value. Therefore, the second sampling value closest to the middle level of the second component can be extracted.
The signal generation device may includes: a first accumulating device that accumulates a plurality of the extracted first sampling values; and a first calculating device that calculates an average of the accumulated first sampling values. Therefore, the first middle level signal representing the actual middle level of the first component can be generated.
The signal generation device may further includes: a second accumulating device that accumulates a plurality of the extracted second sampling values; and a second calculating device that calculates an average of the accumulated second sampling values, in order to generate the second middle level signal. Therefore, the second middle level signal representing the actual middle level of the second component can be generated.
The compensation device may includes: a first shift signal generation device that generates a first shift signal corresponding a difference between the first middle level and the predetermined level; a first addition device that adds the first shift signal to the first middle level signal, thereby generating a first level adjustment signal; and a first subtraction device that subtracts the first level adjustment signal from the first component. Therefore, the first component can be shifted such that its middle level is matched to the predetermined level.
The compensation device may further includes: a second shift signal generation device that generates a second shift signal corresponding a difference between the second middle level and the predetermined level; a second addition device that adds the second shift signal to the second middle level signal, thereby generating a second level adjustment signal; and a second subtraction device that subtracts the second level adjustment signal from the second component. Therefore, the second component can be shifted such that its middle level is matched to the predetermined level.
The information reproducing apparatus may further includes: a clock signal generation device that generates a clock signal, and that supplies the clock signal to the sampling device, the extraction device, and the signal generation device in order to synchronize operations of these devices with a frequency of the clock signal; and a frequency changing device that changes the frequency of the clock signal with a reproduction speed of the information. Therefore, the sampling device, the extraction device, and the signal generation device operate while being synchronized with the frequency of the clock signal. As a result, if the frequency of the clock signal is changed with the reproducing speed, the operations of these devices follow it. Accordingly, if the reproducing speed is changed, noise component contained in the detection signal can be sufficiently eliminated, while maintaining the first component and the second component.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.